1. Field of the Invention
The present invention relates to a method of and an apparatus for preventing diffusion of muddy water in equipment for gathering sand. More particularly, the present invention is concerned with a method of and an apparatus for preventing diffusion of muddy water in a system for gathering sand from the bottom of a lake, river or sea.
2. Description of the Related Art
FIG. 10 illustrates a known system for gathering sand from the sea bottom. A sand gathering boat 1 has a sand pump 5 which can be extended down to the sea bottom through a hose 3 and a sand pipe 4. In operation, wires are paid off from davits 6 on the boat 1 after anchorage of the boat 1 so as to situate the sand pump 5 on the sea bottom. Then, the sand pump 5 is operated to suck the sand from the sea bottom in the form of slurry. The sand pump 5 is operated by a hydraulic or an electric motor directly connected to the sand pump 5.
The water sucked from the sea bottom and containing soil and sand is delivered to a sieving apparatus 7 on the boat 1 through the sand pipe 4 and the hose 3. The sieving apparatus 7 is provided with a metal gauze wire 8 which separates shells and other matters from the water containing sand grains. The water containing sand grains, which has passed the metal gauze wire 8, is introduced into a sand tank 10 through a chute 9. Coarse sand grains 11 which are liable to precipitate are accumulated on the bottom of the tank 10 so as to lay under muddy water 12 comprising suspended fine particles of mud, clay and sand.
A discharge tank 13 is provided on a corner of the sand tank 10, so as to communicate with the sand tank 10 through a gate 14. As the gate 14 is opened, the muddy water 12 flows into the discharge tank 13. Then, as a gate valve 15 is opened, muddy water 12 is discharged to the surface of the sea through a water discharge opening 16 provided at the bottom of the water discharge tank 13 and then through a discharge opening 17 which opens in one side of the shell of the sand gathering boat 1.
In this known sand gathering system, since the muddy water 12 is discharged outboard from the discharge opening which opens in a portion of the boat shell near the sea water level, fine particles of mud, clay and sand contained in the muddy water 12 are diffused into the sea water without sedimentation. The diffused fine particles contaminate the sea water around the boat and are moved by the tide to culture areas for cultivating marine creatures such as shells and fishes, thereby seriously damaging such marine creatures.
In addition to the basic problem described above, the conventional sand gathering system has suffered from a problem in that the suction mouth of the suction opening is clogged with sand even when the suction pump is operated with its full power, particularly when the slurry to be sucked has a high concentration of soil and sand. This problem would be overcome by increasing the power of the suction pump. Such a suction pump having a large power, however, is inefficient from an economical point of view, because only a small part of power is used when the concentration of sand in the slurry is low. Furthermore, the known sand gathering system relying upon a sole suction pump requires a priming of the suction line when the pump is started, thus requiring troublesome work.
Furthermore, fine particles of sand are returned to the sea. Therefore, the sand gathered from the sea bottom by this known sand gathering system exhibits an inferior particle size distribution as compared with sand acquired from land area or from a river bottom, so that the quality of this sand, when used as aggregate for concrete, is inferior to those of sand acquired from land area and river bottom. In order to increase sand gathering capacity, it is necessary to employ a suction pump having a greater capacity. As mentioned above, however, the use of a suction pump having a too large capacity is not recommended from an economical point of view. Thus, it has not been allowed to unlimitedly increase the capacity of the pump on the boat.